System and method for transitioning from one PLL feedback source to another

Abstract

A system and method are provided for switching from one phase-locked loop feedback source to another in a radio frequency (RF) transmitter. The RF transmitter includes a phase-locked loop (PLL) that provides a phase-modulated RF input signal and power amplifier circuitry that amplifies the RF input signal to provide an RF output signal. The PLL includes switching circuitry that couples a feedback path of the PLL to an output of the PLL for open loop operation and couples the feedback path of the PLL to an output of the power amplifier circuitry for closed loop operation. Prior to switching the feedback path from the output of the PLL to the output of the power amplifier circuitry, time alignment circuitry operates to time-align feedback signals from the outputs of the PLL and the power amplifier circuitry such that switching from open loop operation to closed loop operation causes minimal phase disturbance.

Description

FIELD OF THE INVENTION[0001]The present invention relates to phase-locked loops (PLLs) and more particularly to a method for transitioning from one PLL feedback source to another with minimal phase disturbance.BACKGROUND OF THE INVENTION[0002]The Global System for Mobile Communication (GSM) cellular system has recently begun service using a new modulation standard referred to as Enhanced Data rates for GSM Evolution (EDGE). This standard uses 8-Level Phase Shift Keying (8PSK) modulation. Transmitters operating according to the EDGE standard must include a modulator providing both amplitude modulation and phase modulation. To efficiently amplify such modulation, a polar system is desirable.[0003]Polar transmitters may be classified as either open loop or closed loop. In an open loop polar transmitter, a transmit signal is broken into amplitude and phase components. The phase component is provided to the radio frequency (RF) input of a power amplifier. The amplitude component may be u...

AI Technical Summary

Benefits of technology

[0009]The present invention provides a system and method for switching from one phase-locked loop feedback source to another with minimal phase disturbance in a radio frequency transmitter. In general, the radio frequency transmitter includes modulation circuitry that provides a phase modulation signal based on input data, a phase-locked loop that provides a radio frequency (RF) input signal based on the phase modulation signal, and power amplifier circuitry that amplifies the RF input signal to provide an RF output signal. The phase-locked loop includes switching circuitry that couples a feedback path of the phase-locked loop to an output of the phase-locked loop for open loop operation and couples the feedback path of the phase-locked loop to an output of the power amplifier circuitry for closed loop operation. Prior to switching the feedback path from the output of the phase-locked loop to the output of the power amplifier circuitry, time alignment circuitry operates to time-align feedback signals from the outputs of the phase-locked loop and the power amplifier circuitry such that switching from open loop operation to closed loop operation causes minimal phase disturbance.

Problems solved by technology

One issue for an open loop polar transmitter is that amplitude modulation to phase modulation (AM / PM) distortion of the power amplifier varies as the supply voltage varies.

However, if the Voltage Standing Wave Ratio (VSWR) at the output of the power amplifier changes due to variations in load impedance, then the AM / PM distortion also changes.

As a result, the pre-distortion no longer aligns with the AM / PM distortion of the power amplifier.

At higher output power levels, this may become more of an issue because the spectrum and EVM requirements are more stringent.

However, one issue for a closed loop polar transmitter is that strong interference signals present at the antenna are fed back to the PLL along with the output of the power amplifier.

As a result, the output of the polar transmitter is severely distorted.

This phase difference may cause the output of the PLL to deviate from the desired frequency such that the transmitter violates specification.

Thus, if a transmitter fails the −23 dBm limit for any instant in time, the transmitter is out of specification.

As a result, the peak power level at 400 kHz may be exceeded, thereby violating the ETSI switching spectrum specification.

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